Since the start of 2014, there have been several large outbreaks of nomination-based social contagion on social networking sites such as Facebook and Twitter. Unlike much viral online content, which users typically share with their contacts in a spontaneous manner [1]}, [2]}, [3]}, the transmission of nomination-based content follows specific rules. Examples include the `neknomination' challenge [4]}, in which participants posted a video themselves finishing a drink online then nominated contacts to do the same, and the `ice bucket challenge', in which users posted footage of themselves getting doused with icy water [5]}. Users have also nominated others to post `no makeup selfies' [6]}, or to re-activate their first Facebook profile pictures [7]}.
